Pullrod vs. Pushrods

[Bandit_06]

Does anyone have any strong feelings as to whether a pull-rod or push-rod actuated suspension is better suited to an FSAE car?

I was going to start out by listing the reasons I like pullrods better, but I realized, that I'm actually pretty torn...

On one hand, using pullrods accomplishes one major goal: locating the spring/damper assembly very low on the vehicle. While pushrods on the other hand are a heck of alot more reliable, and probably can be constructed with less weight.

Has anyone managed to construct a reliable pull-rod suspension?

[Bandit_06]

Does anyone have any strong feelings as to whether a pull-rod or push-rod actuated suspension is better suited to an FSAE car?

I was going to start out by listing the reasons I like pullrods better, but I realized, that I'm actually pretty torn...

On one hand, using pullrods accomplishes one major goal: locating the spring/damper assembly very low on the vehicle. While pushrods on the other hand are a heck of alot more reliable, and probably can be constructed with less weight.

Has anyone managed to construct a reliable pull-rod suspension?

[Matt Gignac]

We have four main design criteria for determining pushrods vs pullrods: packaging, motion ratios, load pathes and CG height.

Pullrods are easier to package for us, as at the front the dampers end up below the driver's knees, which would otherwise be empty space, and at the rear it makes sense with the way we have other stuff set up.

Since you'll have a shallower angle between the upper a-arm and the pullrod than between a pushrod and the lower a-arm, you'll have less flexibility in terms of motion ratio (namely, you won't get it as high as you'd like).

For load paths, either can be good, depends on your chassis concept, and CG height is a no brainer, except think if there's something heavier you can put in the dampers' place.

Oh, also, with pullrods, no worries about buckling, and if you have carbon fiber pushrods with Al. inserts, if your glue delaminates, you can still have a flange to keep your chassis off the ground.

That's my take on it at least.

Matt Gignac
McGill Racing Team

[Boston]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Matt Gignac:
if you have carbon fiber pushrods with Al. inserts, if your glue delaminates, you can still have a flange to keep your chassis off the ground.
</div></BLOCKQUOTE>

I agree with everything else you said, but can you explain this in more detail?

[Matt Gignac]

The insert was a cylinder with a flange, ID threaded for a rod-end. The CF tube was bonded to the OD of the insert, and the end rested against the flange. In this case, the tube is loaded in compression via the flange, with the glue bond there to keep it all together.

Basically:

(edit: my little ASCII image didn't work)

With pullrods, the glue is in shear, and once that fails, you're effectively driving a sled. We managed to get the pullrods to work this way by going with a larger bond area (thus longer insert), but on our way to testing one day with the car tied down, one of the bonded inserts came out, so that killed our confidence in them, and we decided to analyze the situation a little further before trying that again.

Matt Gignac
McGill Racing Team

[John Stimpson]

I'm not a super chassis/suspension guy, so maybe I'm crazy...

As an engineer, it seems to me that pullrods are superior. The biggest loads on your push/pull rod will be when you hit a bump, in which case the pullrod gets loaded in tension. A pushrods could buckle.

Of course, it is the push/pull rod's job to lift the chassis back up from a jounce condition, meaning a pullrod has to "push" the car back up. However, there are much lighter loads required to "slowly" push the car back up to ride height.

I'm not understanding how you can't achieve any reasonable motion ratio curve with either push or pullrods.

I've read a couple places that you should try to package your damper vertically, so as to avoid cavitation and air bubbles. Again, I'm no expert when it comes to shocks, so I could be off base.

[B Hise]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">I've read a couple places that you should try to package your damper vertically, so as to avoid cavitation and air bubbles. Again, I'm no expert when it comes to shocks, so I could be off base. </div></BLOCKQUOTE>

Most shocks seen in this competition are gas pressurized with a divider piston. This means you can orient the shocks any way you want and if you stay above the mfg. reccomended minimum gas pressure the shock will not cavitate in normal conditions.

Bryan

[jowen]

The shallower angle on a pullrod will also cause a higher force in the pullrod, rocker and rocker mount compared to a more steeper pushrod.

You may also consider the direction of the loads in the a-arms. A pushrod connected to the lower a-arm will lower the compressive force in it and a pullrod will lower the tensile force in the upper.

Anyhow, I think the most important criteria to consider is packaging.

[fade]

the upper a arm could also buckle from the additional loading from the pull rod. Also if you use a rod end for ride height adjustment in you pull rod it could fail from the tensile load. The motion ratio problem is due to vertically mounting the springs and then trying to shorten your lever arm but having to go under the shock. Either method poorly designed can be unreliable. Come up with a goal for your design and rank the importance of the variables, that should get you started developing your own strong feelings. If your still not sure flip a coin.

[Matt Gignac]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by John Stimpson:

I'm not understanding how you can't achieve any reasonable motion ratio curve with either push or pullrods.

</div></BLOCKQUOTE>

Our problem with pullrods was that the angle between the upper a-arm and the pullrod in the front view wasn't very big. Think of the extreme case where the pullrod is in the same plane as the upper a-arm (for instance, a pullrod setup with very small VSAL), your motion ratio is effectively zero. Whereas with pushrods, you have more flexibility as you can vary the angle between the pushrod and the lower a-arm.

Matt Gignac
McGill Racing Team

[KU_Racing]

In this argument, I choose pullrods. Pullrods have quite a few advantages over pushrods, in my opinion.

As far as packaging goes, if the chassis is designed with one system or the other in mind (I would say it should be....) Then it is relatively easy to package a pullrod at an angle very close to the angle a pushrod would see in action.

I like pullrods for their strength- Much better to load a member in tension than in compression. You can make the pullrods either lighter for the same strength level or stronger for the same weight level as a pushrod.

Also, the CG gains from placing the shocks under or close to under the chassis are obvious.

One small side benefit is that if a pullrod fails, there are no members that are going to go into the ground (which has caused FSAE cars to roll in the past) or through the shielding and into the driver's compartment.

[flavorPacket]

I don't understand why you guys are saying you can't get a good motion ratio from a pullrod system. I was able to get a nice linear .93 damper/wheel with minimal headache.

And I don't think the cg benefits are that compelling. Instead of having dampers (which weigh all of 2 lbs) under the driver, why not have the 60+ lbs of driver's legs under the dampers?

[Gareth]

Historically UW has run pullrod suspensions for the CG benefits and the buckling requirements. We tried a few different configurations, each with its own pros and cons. Last year we switched to pushrods for a variety of reasons. The first was simply that we were tired of lying on the ground to work on the springs/dampers. As silly as that sounds, our philosophy was that we could go faster by making more setup changes and tuning the car than by putting the mass a little lower. After the initial concept we discovered massive hub-to-hub stiffness gains, but that's very dependent on your chassis concept. The other benefit was that it moved around the forces in the control arms, making them more evenly distributed. Ultimately we came to a more efficient structure that also gave us more room to package a kinematically better anti-roll system.

I don't think there is a clear winner between the two, but it's definitely good to have the various compromises in mind when making your decision. Of course you can achieve good kinematics with both, provided your other parts don't get in the way.

[C.Zinke]

Overall, i think this is a thread with a lot of good arguments. Furthermore i agree that the decision is depending on your kinimatc, structure and package.

I like to add that a pushrod and a pullrod produce reaction forces. But normally your lower A-arm is loaded higher. So it stands this reaction forces anyway. If you use a pullrod, then you also have to use big tubes for the upper A-arm.

Furthermore you reduce the risk of buckling (rear tube of front lower A-Arm) during braking with a pushrod

We use Pushrod. So its possible to build the upper A-arms this way:

http://www.lionsracing.de/component/option,com_ponygall...func,detail/id,1171/ (http://www.lionsracing.de/component/option,com_ponygallery/Itemid,68/func,detail/id,1171/)


With a pullrod and small upper A-arms this can happen:

http://evilengineering.com/gallery/v/UMDracing/FSAE/AUS06/P3032679.jpg.html

[D J Yates]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by C.Zinke:
I like to add that a pushrod and a pullrod produce reaction forces. </div></BLOCKQUOTE>

I was getting worried there that everyone had forgoton newton's third law. Further more, since the angle between pullrod/pushrod is rarely greater than 45 degrees to the horizontal, the reaction in the wishbones is greater than the normal load on the tyre.

Another thing to consider is the cyclic nature of the tyre normal load. Since only cycling tensile loads reduce fatigue life, ask yourself would you rather have to replace a pullrod or the lower wishbone of a pushrod assembly.

With a carefully designed pullrod you could garauntee the failure mode of your suspension and the subsequent damage it could do. Say that your inexperience/rubbish driver hit a large curb head on, if your suspension could withstand the shock loading chances are your wheel couldn't. Would you rather replace a simple pullrod or an expensive ultra light weight wheel. If the pushrod was a thin walled tube with a tether running down thinside, the pullrod could fail in tension and allow the wheel to ride up over the curb and save the wheel, but the tether would stop the car bottoming out when it landed.

Personally i think that pullrod or pushrod choice should be determined by the location of the steering joint. If your pullrod/pushrod and steering link connect to the upright at the same end (steering from top with pullrod, steering from bottom with pushrod) you are free to move the ball joint of the other wishbone, giving yourself easyilly adjustable caster and kpi without any effect on steering geometry or motion ratios.

[Iloper]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by John Stimpson:
... meaning a pullrod has to "push" the car back up. However, there are much lighter loads required to "slowly" push the car back up to ride height ...

</div></BLOCKQUOTE>

You miss the point here, a pullrod never pushes the car, hence the name. The only time a pullrod gets buckeling forces is when the wheel is in the air and gravity is forcing it down.

Whe had quite som problems with this, but only when the car is in the pit, when the vehicle is of the ground and an exhausted needs to rest, he sometimes wants to sit on a tire. Or one other time when our brake discs melted and the car took a halt on the track. When it was time to lift it up, the tyres were glued to the ground and Bamm! both front pullrods looked like a banana.

We have been using 6mm steel rods, we're changing that to tubes now.

Per
CFS

[jowen]

Per, how could your brake disc melt?! Shouldn't the brakes be fading a long time before that happends?

[Iloper]

I didn't drive, so I don't know if they faded. The discs didn't realy melt, just deformed heavily. The problem was dirt in the brake fluid, the pistons didn't retract enough, together with cermic coated alu (6082) discs. The coating is still there though.

Per
CFS

[js10coastr]

Chariots of Fire by Steve Matchett has a pretty good section arguing the points of each, and why F1 uses pushrods now.

[TG]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by js10coastr:
Chariots of Fire by Steve Matchett has a pretty good section arguing the points of each, and why F1 uses pushrods now. </div></BLOCKQUOTE>

I'm pretty sure that was mostly for aero reasons. F1 is heavily dependant on the aerodynamics of their vehicles. This is plainly evident in the fact that they are now compromising on sub-optimal suspension geometry in the zero-keel designs to improve aerodynamic efficiency.

[VFR750R]

This is probably accurate. Sportscars, Nascars, IRL cars ect all compromise mechanical grip in their suspension settings to optimize aero.

[js10coastr]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by TG:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by js10coastr:
Chariots of Fire by Steve Matchett has a pretty good section arguing the points of each, and why F1 uses pushrods now. </div></BLOCKQUOTE>

I'm pretty sure that was mostly for aero reasons. F1 is heavily dependant on the aerodynamics of their vehicles. This is plainly evident in the fact that they are now compromising on sub-optimal suspension geometry in the zero-keel designs to improve aerodynamic efficiency. </div></BLOCKQUOTE>

No... there are other reasons(at the time). Just read the book, it's pretty entertaining.

...and I screwed up the title, it's "The Chariot Makers"

[John Stimpson]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Iloper:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by John Stimpson:
... meaning a pullrod has to "push" the car back up. However, there are much lighter loads required to "slowly" push the car back up to ride height ...

</div></BLOCKQUOTE>

You miss the point here, a pullrod never pushes the car, hence the name. The only time a pullrod gets buckeling forces is when the wheel is in the air and gravity is forcing it down.

Whe had quite som problems with this, but only when the car is in the pit, when the vehicle is of the ground and an exhausted needs to rest, he sometimes wants to sit on a tire. Or one other time when our brake discs melted and the car took a halt on the track. When it was time to lift it up, the tyres were glued to the ground and Bamm! both front pullrods looked like a banana.

We have been using 6mm steel rods, we're changing that to tubes now.

Per
CFS </div></BLOCKQUOTE>

Hmmm... Am I on crack? With a pullrod, we can agree that when you jounce the suspension, the pull rod is in tension, right?

Well, if you were hypothetically on the bumpstop and the susp is going to rebound, isn't it the spring that must generate the force to lift the chassis back up? If thats the case, doesn't the spring have to communicate with the wheel ultimately through the pullrod? Doesn't that mean the pullrod must push that corner of the chassis up?

[Iloper]

Sorry to inform you John, but you are still lost in the force directions, don't worry I've been there aswell.

When you compress the spring, the wheel has to PULL in the pullrod (you push the chassi down, wheel goes "up"). When you are in rebound, the spring PULLS the wheel "down" and therefore it also lifts the chassi up (like in the case you just described), it also pulls in the pullrod.

Still, the only time you get pushing forces in a pullrod is when gravity whats to force the wheel down faster than the spring whants to pull it down. Or when the car is in the air, and a fat teammate whants to sit on the wheel.

So, Yes! You are on crack! But I believe you'll get over the rush, and soon be pulled back on earthhttp://fsae.com/groupee_common/emoticons/icon_smile.gif

/Per